Recognition sites (receptors) present in the central nervous system are physiologically modulatyed by endogenous transmitters and modulators. Furthermore, it is believed that many psychotropic drugs exert their therapeutic actions by affecting these receptors. Several receptor systems are currently under study including receptors for benzodiazepines, tricyclic antidepressants, adenosine and the neuropeptide, cholecystokinin. The role and regulation of these systems are investigated by: 1) Characterization of these recognition sites. 2) Characterization of endogenous modulators of these receptor sites. 3) Development of appropriate in vitro models for these receptors. 4) Structure-activity studies of chemically modified substances which are related to the naturally-occuring ligands of these receptor sites. 5) Behavioral and pharmacologic experiments in animals combined with an examination of subsequent alterations in the character of these receptor sites. 6) Genetic studies in inbred strains of rats and mice coupled with examination of receptor changes as they correlate to gross behavioral manifestations of these genetic traits. 7) Clinical studies on receptors present in readily accessible peripheral tissues of man.